thermodynamics – Is work always done to achieve motion against an opposing force?

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thermodynamics – Is work always done to achieve motion against an opposing force?

You are right, and the book is wrong – or at least unclear.

The most general definition of work is $W = \int \vec{F} \cdot d \vec{l}$, where $F$ is the force applied to an object and the integral is over the path along which the object is displaced; as you rightly note, there is no opposing force involved at all! The only thing you need to know to calculate the work being done on some system is the force applied to it, and the path that it follows.

The definition given in the book is therefore confusing at best, and for many situations not correct. However, since the book is on physical chemistry, the author might have a particular type of system in mind (and probably not a ball accelerated in vacuum).

If instead you think of a gas confined to a volume with a movable piston, then to push the piston (and compress the gas) you need to apply a force that exceeds the pressure of the gas. Because of this opposing force, you cannot move the piston without doing work – the force you apply must be at least $F = pA$ where $p$ is the pressure of the gas and $A$ the area of the piston where the force is applied. If the pressure were zero, then it would be possible to move the piston without doing any work, just like in the case of a ball in vacuum. For this limited type of system, it therefore does make some sense to think of work as ‘being done to overcome an opposing force’.

I don’t know much about physical chemistry, but my guess would be that this is the kind of system the author had in mind.

thermodynamics – Is work always done to achieve motion against an opposing force?
Figure: Gas in cylinder with movable piston. From https://physicslabs.ccnysites.cuny.edu/img/tutorial-ideal-gas-piston-1.jpg

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